This invention relates to welding aluminum-magnesium-silicon-copper alloys wherein aluminum comprises more than 90% of the composition.
Because of increasing emphasis on reducing weight in automobiles, major efforts have been made to develop aluminum alloys which are suitable for use as body sheet and structural frame members. Those efforts have led to the development of new 6XXX magnesium silicide alloys, such as 6009, 6010 and 6013, for example, which have desirable characteristics and properties for structural applications such as relatively high mechanical properties, can be cold-formed, as relatively economical to fabricate in extruded or sheet form and have excellent corrosion resistance. In addition, these alloys are suitable for welding by gas tungsten arc (GTA) or gas metal arc (GMA) welding methods when using 4043, 4145, 4047 or other aluminum-silicon alloy filler metals. Although welds made using 4XXX alloys for filler metals are satisfactory in many instances, welds using 5XXX alloy filler metals are generally preferred for structural applications because of their relatively high weld strength and ductility. These newly developed 6XXX alloys were evaluated for joining by arc welding with a conventional 5356 alloy filler metal, and it was discovered that satisfactory welds could not be consistently produced. Cracking along the fusion line between the weld and parant metal was visually evident in some welds after applying die penetrant. In others, the cracks were not visually evident but their presence as small, incipient cracks was evident from the premature failure of test specimens in tension and elongation well below expected values.
It is believed that cracking adjacent to the welds when using 5XXX filler metal is a function of the amount of copper present in the base alloy, since the newly developed alloys such as 6009, 6010 and 6013 have a higher copper content than conventional, commercial 6XXX alloys available heretofore. To enhance the use of 6XXX copper-bearing alloys and take advantage of their many desirable properties and characteristics, it would be advantageous to provide a filler metal which could be used with such alloys without generating cracks in the fusion zone between the weldment and parent metal.